1. Field of the Invention
The present invention relates to a process for concentrating an at least partially crystalline solid containing at least one zeolite in a mixture comprising at least one auxiliary, for example a template compound, and said solid. In particular, the process comprises ultrafiltrating the mixture in a step (II) to divide the mixture into a retentate and a permeate, the solids content in the retentate being higher than that in the mixture and the solids content in the permeate being lower than that in the mixture. This process procedure allows auxiliaries, in particular template compounds, present in the permeate to be recycled into a crystallization step (I) upstream of step (II).
2. Description of the Background
Integrated processes for producing zeolite-containing shaped bodies are described, for example, in commonly assigned documents, in particular in WO 98/55229. This document focuses on the selection of the binder which allows compaction of the zeolite-containing solid to produce a shaped body. WO 98/55229 does not disclose any processes for concentrating the solid present in the mother liquor which are not covered by the conventional processes of filtration and/or centrifugation.
U.S. Pat. No. 6,106,803 describes a process for preparing titanium silicalite granulates comprising crystallizing a synthesis gel (=synthesis mixture; Si and Ti source, templating agent, base and water) under hydrothermal conditions, thereby producing a zeolitic suspension, and subjecting said suspension to spray-drying or fluidized-bed spray granulation drying after optionally concentrating and/or adding further substances. The solids content in the mixture prior to the spray drying step is in the range near 10%. Such a comparatively low solids content eventually leads to an unnecessarily reduced catalytic activity per unit mass in the shaped body, in particular when additives are added to the mixture for granulating.
EP-B 0 638 362 relates, inter alia, to a method for preparing a titanium silicalite catalyst, and here in particular the agglomeration of the primary particles, i.e. the microparticles which form during the crystallization step of the titanium silicalite synthesis. This agglomeration is achieved by reducing the pH of the solution containing the primary particles (zeolitic suspension) to values ranging from pH 5 to pH 10. The agglomeration forms part of an integrated process in which (i) firstly, the primary particles are prepared from a synthesis mixture of the prior art, (ii) subsequently, the secondary particles are agglomerated as mentioned above by reducing the pH, and (iii) the secondary particles are finally calcined. However, with respect to concentrating primary or secondary particles prior to agglomeration and/or recovering components of the mother liquor, EP-B 0 638 362 does not offer any teaching beyond that of the prior art.
U.S. Pat. No. 4,701,428 likewise addresses the problem of agglomeration in a mixture containing zeolitic microcrystals (here: smaller than 5 μm). The problem is solved by a special procedure for agglomerating titanium silicalite. This procedure comprises adding titanium silicalite crystals to a solution containing tetraalkyl orthosilicates at defined temperatures and quick drying. This document likewise does not offer any teaching beyond that of the prior art with respect to concentrating titanium silicalite crystals and/or recovering components of the mother liquor. A similar process is described in EP-B 1 106 576.